Naturally present, in the skin, hyaluronic acid is known for its viscoelastic properties and also for its very great propensity to absorb water. Its properties largely explain the elasticity of skin. Its biocompatibility, tolerance and lack of toxicity are such that, for more than a decade, this molecule has had applications in the medical and cosmetic fields, and in particular for filling wrinkles. Thus, the injection of a crosslinked polysaccharide hydrogel into the dermis at the wrinkles to be treated helps to lessen, or even eliminate, the local sag in the structure of the dermis that is a wrinkle.
In general, the polysaccharide and more particularly hyaluronic acid, is used in a crosslinked form owing to the greater resistance in this particular form to degradation and to heat.
These crosslinked polysaccharide gels may be obtained by various methods of preparation. These methods generally require two main steps, the first consisting in hydrating the polysaccharide in question, to convert it into an aqueous gel, and the second aiming to crosslink said aqueous gel in the presence of an agent typical of causing the crosslinking thereof.
Conventional crosslinking methods for hyaluronic acid are usually carried out at a temperature of around 40° C., or even 50° C., for a time of less than 3 hours, thereby making it possible for the polysaccharide gel to be crosslinked satisfactorily. It is for this reason that tins method of preparation is widely used in the industry.
As illustrations of these processes, the processes described in US 2006/0105022, which comprises the use of a mixture comprising at least 10% hyaluronic acid, a crosslinking agent and water under acid or basic conditions. In WO 2006/056204, which comprises a step of treating the crosslinked hyaluronic acid gel with divinyl sulphone and in US 2007/0036745, which results in a cohesive gel starting from a hyaluronane polymer crosslinked with divinyl sulphone (DVS) may especially be mentioned.
For obvious reasons, it is a constant objective to improve the mechanical properties of the hydrogels for applications in the medical and cosmetic fields.